HURRICANE EDOUARD (AL062014) 11 – 19 September 2014
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, NATIONAL HURRICANE CENTER TROPICAL CYCLONE REPORT HURRICANE EDOUARD (AL062014) 11 – 19 September 2014 Stacy R. Stewart National Hurricane Center 10 December 2014 EDOUARD AS SEEN FROM THE INTERNATIONAL SPACE STATION ON 17 SEPTEMBER 2014. (IMAGE COURTESY OF NASA) Edouard was a category 3 hurricane (on the Saffir-Simpson Hurricane Wind Scale) that remained over the open Atlantic Ocean during its lifetime. Edouard was the first major hurricane to develop in the Atlantic basin since Hurricane Sandy of 2012. Multiple research missions, sometimes simultaneous, were conducted in Edouard by NOAA and NASA aircraft, including the first-ever release of an unmanned aerial vehicle into an Atlantic tropical cyclone. Hurricane Edouard 2 Hurricane Edouard 11 – 19 SEPTEMBER 2014 SYNOPTIC HISTORY Edouard developed from a westward-moving tropical wave that exited the coast of western Africa late on 6 September. The wave was accompanied by a broad low pressure system and disorganized deep convection, which fluctuated significantly during the next four days as the disturbance moved west-northwestward over the far eastern tropical Atlantic. By late on 10 September, however, convection began to increase near the center of the surface low when the system was located well to the west of the Cape Verde Islands. By 1200 UTC 11 September, deep convection had become sufficiently organized to designate the system as a tropical depression about 720 n mi west of the Cape Verde Islands. The “best track” chart of the tropical cyclone’s path is given in Fig. 1, with the wind and pressure histories shown in Figs. 2 and 3, respectively. The best track positions and intensities are listed in Table 11. The depression turned toward the northwest and maintained that general motion for the next five days as the cyclone moved around the southwestern periphery of a deep-layer subtropical ridge that extended across the central and eastern Atlantic Ocean along 30O- 32O N latitude. Upper-level winds and sea-surface temperatures were quite favorable for development, but the surrounding environment of drier-than-normal mid-level air likely inhibited the intensification process. Slow but steady strengthening occurred while the cyclone moved northwestward, with the system becoming a tropical storm early on 12 September and a hurricane early on 14 September. Edouard became a major hurricane early on 16 September, reaching its peak intensity of 105 kt at 1200 UTC that morning when it was located about 360 n mi east of Bermuda, making it the first major hurricane to develop in the Atlantic basin since Hurricane Sandy of 2012. However, Edouard’s status as a major hurricane was short-lived. The inner-core deep convection eroded significantly and a weakening trend began almost immediately, likely caused by an eyewall replacement cycle (Fig. 4) and perhaps cold upwelling/mixing of at least 7O C beneath the hurricane (Fig. 5). Late on 16 September, Edouard moved northward while weakening, and then turned northeastward and accelerated on 17 September ahead of an approaching mid-latitude trough and associated frontal system. The next day, Edouard turned eastward and began to rapidly weaken as the hurricane became embedded in the mid-latitude westerlies and encountered strong westerly vertical wind shear. Edouard weakened to a tropical storm late on 18 September, and degenerated into a strong post-tropical cyclone early on 19 September about 400 n mi west of the western Azores Islands. The remnant low moved southeastward and then 1 A digital record of the complete best track, including wind radii, can be found on line at ftp://ftp.nhc.noaa.gov/atcf. Data for the current year’s storms are located in the btk directory, while previous years’ data are located in the archive directory. Hurricane Edouard 3 southward over the next two days before merging with a frontal system early on 21 September several hundred n mi south-southwest of the Azores Islands. METEOROLOGICAL STATISTICS Observations in Edouard (Figs. 2 and 3) include subjective satellite-based Dvorak technique intensity estimates from the Tropical Analysis and Forecast Branch (TAFB) and the Satellite Analysis Branch (SAB), and objective Advanced Dvorak Technique (ADT) estimates from the Cooperative Institute for Meteorological Satellite Studies/University of Wisconsin- Madison. Observations also include flight-level, stepped frequency microwave radiometer (SFMR), dropwindsonde observations, and Coyote unmanned aerial vehicle (UAV) flight-level data from flights conducted by the NOAA Aircraft Operations Center (AOC) WP-3D Hurricane Hunter aircraft, and dropwindsonde observations from flights conducted by the NASA AV-6 Global Hawk unmanned aircraft as part of the Hurricane and Severe Storms Sentinel (HS3) research program. Data and imagery from NOAA polar-orbiting satellites including the Advanced Microwave Sounding Unit (AMSU), the NASA Tropical Rainfall Measuring Mission (TRMM), the European Space Agency’s Advanced Scatterometer (ASCAT), and Defense Meteorological Satellite Program (DMSP) satellites, among others, were also useful in constructing the best track of Edouard. There were no ship reports of tropical-storm-force winds associated with Edouard while it was a tropical cyclone. Winds and Pressure Edouard’s estimated peak intensity of 105 kt at 1200 UTC 16 September is based on a blend of satellite intensity estimates of T5.5/102 kt from TAFB and T5.7/107 kt from UW-CIMSS ADT. The minimum pressure of 955 mb at 1200 UTC 16 September is based on the Knaff-Zehr- Courtney (KZC) pressure-wind relationship corresponding to an intensity of 105 kt. The NOAA WP-3D Hurricane Hunters aircraft conducted eight research missions into or around Hurricane Edouard during the period of 11-19 September, generally during daylight hours between 1200-2100 UTC. Those missions resulted in nine center fixes that occurred at either the 750 mb or 700 mb pressure levels (~8,000 and ~10,000 ft, respectively). Maximum flight-level winds of 115 kt and 114 kt were observed at 1622 UTC and 1722 UTC UTC, respectively, on 15 September. The strongest SFMR surface winds measured were 94 kt at 1804 UTC 15 September and 92 kt at 1620 UTC 16 September. The strongest surface winds measured by a dropwindsonde in the eyewall were 103 kt at 1804 UTC 15 September and 98 kt at 1413 UTC 16 September. The lowest pressure measured in the eye of Edouard by a NOAA dropwindsonde was 957 mb at 1652 UTC 16 September. However, the dropwindsonde also reported a surface wind of 11 kt, so the minimum central pressure at that time is estimated to be 956 mb. Hurricane Edouard 4 The NASA AV-6 Global Hawk similarly conducted eight research missions into and around Hurricane Edouard on 11, 14, and 16-19 September. Unlike the NOAA WP-3D aircraft, the Global Hawk aircraft overflew Edouard at altitudes in excess of 60,000 ft and therefore did not make any eyewall penetrations. The unmanned aircraft did, however, release more than 340 dropwindsondes, including a few into the eye of the hurricane. During an overflight mission at 0612 UTC 17 September, a dropwindsonde was released from an altitude of about 62,000 feet into the inner edge of the southern eyewall during a south-to-north pass. The instrument rotated counter-clockwise through the eastern semicircle of the eyewall, measuring a peak wind speed of 95 kt just a few hundred feet above the surface, a surface wind speed of 90 kt, and a sea- level pressure of 963 mb (Fig. 6). For the first time ever, an unmanned aircraft was deployed by a NOAA WP-3D manned aircraft directly into a hurricane (Figs. 7 and 8). NOAA released a total of four Coyote UAVs into Hurricane Edouard during flights conducted on 15-17 September 2014 while operating out of Bermuda (Fig. 9). The Coyotes sent back important meteorological data from both Edouard’s eye and surrounding eyewall. Before the system failed after completing a counter-clockwise flight path that lasted 28 minutes on 16 September, the Coyote reported a maximum flight-level wind of 100 kt at 1432 UTC at an altitude of about 3,200 ft ASL (971 m) when the small UAV was located in the southwestern eyewall. Additional information on missions conducted in Hurricane Edouard by NOAA AOC, including deployment of the Coyote UAV, and also by the NASA Global Hawk unmanned aircraft can be found in the References section at the end of this report. CASUALTY AND DAMAGE STATISTICS There were no reports of damage or casualties associated with Edouard. FORECAST AND WARNING CRITIQUE The genesis of Edouard was not forecast particularly well in the short term (Table 2). An area of disturbed weather was introduced into the Tropical Weather Outlook (TWO) with a 30% (medium) chance of formation in five days at 0000 UTC 6 September, 132 h prior to genesis. The system was introduced in the 48-h forecast period with a low genesis probability (<30%) and the probability was increased to the medium chance category (30-50%) 6 h later. However, the 48-h probabilities were decreased to the low chance category 36 h before genesis occurred; similarly, the 5-day genesis probabilities were also decreased to the medium chance category 36 h before genesis occurred. Although the five-day probabilities were increased back into the high category of development 30 h before Edouard formed, the 48-hour genesis probabilities never reached the high category. Hurricane Edouard 5 A verification of NHC official track forecasts (OFCL) for Edouard is given in Table 3a. Official forecast track errors ranged from 19% - 35% lower than the mean official errors for the previous 5-yr period throughout the entire 120-h forecast period. A homogeneous comparison of the official track errors with selected guidance models is given in Table 3b.